A wide variety of useful polymers are obtained by polymerizing a vinyl compound through an addition polymerization, e.g., a radical, a cationic or an anionic polymerization. This polymerization proceeds through a chain reaction, so that, when the vinyl compound contains chain terminating or chain transferring compounds as formed during the production of the vinyl compound, adverse effects are caused such as retardation of the polymerization reaction and decrease of the molecular weight of the obtained polymer. Many of these polymerization inhibiting substances give adverse effect on the polymerization reaction even if their amount is very small. Therefore, it is needed to completely eliminate the above polymerization inhibiting substances, for imparting excellent polymerizability (polymerization activity) to the vinyl compound.
A polymerization inhibitor is often added to a vinyl compound having a radical polymerization activity during or after the production thereof in order to inhibit the polymerization during the production, storage and transportation. Therefore, although a polymerization initiator is often added in an amount sufficient to countervail the effect of the polymerization inhibitor in the polymerization of the vinyl compound, it is preferred that the polymerization inhibitor be removed from the vinyl compound prior to the polymerization, for obtaining a polymer of high quality.
However, it is often difficult to remove the polymerization inhibitor and other polymerization inhibiting substances from a vinyl compound containing an oxygen, a nitrogen or a sulfur atom, i.e., a vinyl compound having a polar group (polar vinyl compound). When the polar vinyl compound is purified by distillation, it is generally required to withdraw the same in vacuum and simultaneously at a temperature not lower than the melting point thereof because the boiling and melting points of the polar vinyl compound are relatively high, so that the operating conditions of the distillation are often extremely restrained. Further, there is the danger that the polar vinyl compound is polymerized during the distilling operation. Therefore, the practical use in industry of the distillation for the purification of the polar vinyl compound involves problems.
Also, the polar vinyl compound is purified by recrystallization. This requires purification of the recrystallization solvent and drying of the product. Further, there is the danger that a polymer of the polar vinyl compound is formed by local heating in the drying step. Therefore, the industrial purification of the polar vinyl compound by recrystallization involves economic disadvantages.
The method of effecting adsorption and removal of polymerization inhibiting substances by passing a solution of a vinyl compound, especially a polar vinyl compound through a column packed with an ion exchange resin or active carbon does not require separation of the vinyl compound from the treated solution and hence is advantageous in the direct use thereof in polymerization, so that it is widely utilized in the purification of the polar vinyl compound. However, a solvent suitable for use not only in adsorption operation but also in polymerization reaction must be selected taking the availability and economy thereof into account, and it has practically been unfeasible to find a solvent meeting all of these requirements. Further, it is necessary to conduct regeneration or exchange of an adsorbent, and this involves inconvenience in the execution on an industrial scale. Still further, a crystallized vinyl compound is sometimes demanded when the compound is transported to a remote place, when it is stored in a cold place, or depending on the use thereof. In that case, the same problems as mentioned with respect to the above purification method may be involved.
Japanese Patent Publication No. 56(1981)-41282 discloses a process comprising applying a high pressure to a mixture to thereby crystallize a component and separating the crystals from a liquid phase under pressure, generally known as the pressure crystallization process. This pressure crystallization process is known to be applicable to separation of position isomers of xylene, naphthalenes and cresol and to separation of alkylated phenols from phenol alkylation reaction mixtures, as disclosed Japanese Patent Laid-open Publication Nos. 62(1987)-209034, 1(1989)-250329 and 4(1992)-120027. However, there has been no disclosure regarding the application of the pressure crystallization process to the vinyl compound for imparting excellent polymerizability to a polymerizable vinyl compound.
For example, it is known hat the N-vinylcarboxamide can be produced by first synthesizing an N-(1-alkoxyethyl)carboxamide from a carboxamide, acetaldehyde and an alcohol and then performing thermal or catalytic decomposition of the N-(1-alkoxyethyl)carboxamide. However, the properties, especially the boiling point and solubility of the N-vinylcarboxamide are very close to those of the unreacted carboxamide and N-(1-alkoxyethyl)carboxamide, so that their separation is not easy, although some methods have been proposed.
Japanese Patent Laid-open Publication No. 61(1986)-286069 discloses an extractive separation using water and an aromatic hydrocarbon because the mingling of formamide as the unreacted starting material into the N-vinylformamide cannot be avoided in the distillation.
Another known advantageous process for producing the N-vinylcarboxamide comprises first synthesizing an ethylidenebiscarboxamide from acetaldehyde and a carboxamide and decomposing the ethylidenebiscarboxamide into the carboxamide and the N-vinylcarboxamide. This process produces the carboxamide and the N-vinylcarboxamide which have similar properties in equimolar amounts, and their separation is very difficult. Japanese Patent Laid-open Publication Nos. 63(1988)-132868 and 2(1990)-188560 and U.S. Pat. No. 4,401,516 respectively disclose a technique of cooling crystallization from an organic solvent mixture, a technique of extraction with an aqueous solution of an inorganic salt and an aromatic hydrocarbon and a technique of extractive distillation with a polyhydric alcohol.
However, it is difficult to obtain a satisfactorily purified N-vinylcarboxamide irrespective of any of the above techniques applied. Further, the extraction technique requires expensive organic solvents, and thus equipment for recovering and purifying the same. Still further, the N-vinylcarboxamide is relatively unstable in water, so that there is the danger that the N-vinylcarboxamide undergoes hydrolysis during the extracting operation. Thus, the extraction technique is not an industrially satisfactory method. On the other hand, the technique of cooling crystallization from an organic solvent involves the same problems concerned with the use of the organic solvent as in the extracion technique, and in addition disadvantageously requires a drying step and involves the problem that there is the possibility of thermal polymerization of the N-vinylcarboxamide. With respect to the extractive distillation technique, an organic solvent is used, so that the reflux ratio must be increased for attaining desired rectification effect, thereby necessitating heating of the N-vinylcarboxamide for a prolonged period of time.
All of the above conventional techniques disclosed cannot ensure efficient stable separation of a highly purified polar vinyl compound such as N-vinylcarboxamides having high polymerizability on an industrial scale.